In a magnetic recording device in which a read head comprises a magnetoresistive (MR) sensor, there is a constant drive to increase recording density. One trend used in the industry to achieve this objective is to decrease the size of the MR sensor. Typically, the sensor stack has two ferromagnetic layers that are separated by a non-magnetic layer. One of the ferromagnetic layers is a reference or pinned layer wherein the magnetization direction is fixed by exchange coupling with an adjacent antiferromagnetic (AFM) pinning layer. The second ferromagnetic layer is a free layer with a magnetization that rotates in response to external magnetic fields, and is rotated towards either parallel or anti-parallel to the magnetization in the pinned layer to read out the local orientation of magnetic moment in the recording media. When passing the MR sensor over a recording medium at an air bearing surface (ABS), the free layer magnetic moment will rotate according to the local magnetic field generated by the recording media. By processing the angle of rotation as a function of location on the media, the data pattern recorded on the media can be decoded. A MR sensor may be based on a tunneling magnetoresistive effect where the two ferromagnetic layers are separated by a thin non-magnetic dielectric layer. A sense current is used to detect a resistance value, which is lowest when the moments from the two layers are parallel to each other and is the highest when the two moments are anti-parallel to each other. In a current perpendicular-to-plane (CPP) configuration, a sense current is passed from a top shield through the sensor layers to a bottom shield, or in the reverse direction.
A longitudinal biasing scheme is typically used in a read head design to keep the free layer in a stable orientation in the absence of the external magnetic field. Bias films of high coercivity or soft bias also known as junction shields, are abutted against the edges of the MR sensor and particularly against the sides of the free layer. By arranging for the flux flow in the free layer to be equal to the flux flow in the adjoining hard bias layer, the demagnetizing field at the junction edges of the aforementioned layers vanishes because of the absence of magnetic poles at the junction. As the critical dimensions for MR sensor elements become smaller with higher recording density requirements, the free layer becomes more volatile and more difficult to bias. Top and bottom magnetic shields with in-plane magnetization are often used to ensure the MR sensor will only respond to a local magnetic field. However, free layer magnetization is sensitive to domain wall motion in the bottom and top shield, which may lead to increased noise, reducing the signal to noise (SNR) ratio of the reader sensor and cause failure in decoding data from the media.
In recent years, 2DMR configurations have become attractive from an areal density improvement standpoint. However, shield stability is more difficult to control in 2DMR schemes because of a requirement to shrink reader-to-reader spacing (RRS), and in view of repeated thermal treatments during recording head fabrication that can readily change the magnetization orientation in the shields. Shield instability will directly translate into reader instability and will adversely impact SNR and bit error rate (BER). A new read head structure is needed wherein shield stability is improved while maintaining acceptable SNR and BER.